1. Field of the Invention
The present invention is broadly concerned with improved, stand-alone pump stands for controlling the flow rate of a liquid from a liquid source. More particularly, the invention is concerned with such pump stands, and corresponding methods, wherein the flow rate of liquid is rapidly approached and maintained at or about a preselected setpoint flow rate.
2. Description of the Prior Art
In many agricultural applications, seeds are coated with various liquid chemicals prior to planting. Such liquids may include pesticides, growth stimulants, or plant nutrients, and these liquids have a variety of different physical characteristics such as viscosities and drying rates. Seed treaters have been developed for coating large volumes of seed on an industrial basis. Generally, such treaters include a seed inlet, a sprayer or other device for applying coating liquids to the surfaces of the seeds, and some sort of rotating drum or the like to assure uniform seed coating. As such, it is necessary to deliver the treating liquids to the application device at a uniform flow rate. That is to say, if the liquids are delivered at variable flow rates, then the seeds will be differentially coated, depending upon the instant at which the seeds were treated with the liquid.
There are two basic types of pumping systems for handling and pumping of seed treating liquids delivered to downstream seed treaters. Most often, these systems are mounted on a stand or other support structure separate from the seed treaters. These systems include mix tank(s), pump(s), tubing system(s), and various types of monitoring and control equipment.
The first type is a “manual” system that uses a controller of some type to drive the pump motor(s) based upon a proportional signal. For example, the pump motor(s) can be operated at 50% of the maximum speeds thereof. If the operator needs to adjust the liquid flow rate exiting the pump, the flow rate must be manually adjusted using the controller. In these types of systems, the flow rate is typically determined either by an in-line flow meter, or a “catch and time” technique wherein the volume of liquid delivered over a selected period of time is measured, and the flow rate is thus determined. Some of these manual systems also have the ability to read a signal from a downstream seed treater to either turn on the pump(s), or to terminate the operation thereof. However, none have the ability to automatically adjust liquid flow rate over time. Furthermore, these systems do not have any means of automatically reporting the volume of liquids pumped, so the operator must either use a flow meter totalizer or some sort of mass balancing to calculate the total chemical usage for a given time period.
The second type of system is an automated or automatic system, and is generally plug-connected to a PLC-based controller that performs all of the logical steps for pumping operations.
In these systems, an operator may set a desired flow rate, and the pump speed will automatically adjust to meet that desired flow rate, based upon information received by the PLC from a connected flow meter. This is generally done via a PI (proportion-integral) or PID (proportion-integral-derivative) software control loop. These systems also have automatic reporting capabilities that show total chemical usage.
In many instances, upgrades are available for the manual and automatic systems, which allow adjustment of the liquid flow rate during operation. These upgrades allow an operator to offset via a multiplier any consistent inaccuracies that the flow meter may display. Such inaccuracies occur quite often with the use of standard volumetric/electromagnetic flow meters normally employed. All of the known manual system upgrades provide only a display device, and do not provide any control function. Moreover, they have only one adjustable offset or multiplier per flow meter.
As such, there are presently no “stand-alone” liquid pump stands which allow the operator to choose a desired setpoint flow rate, with the on-board controllers serving to automatically maintain the set point flow rate via adjustment of pump speeds. Moreover, none of the known “automatic” pump stands makes use of multiple PI or PID control loops to accomplish fast pump speed changes and steady pump speed control. Additionally, all adjustable flow rate displays for “stand-alone” pump stands have only one available adjustment for each flow meter. This means that if an operator wishes to run a different liquid through the same flow meter, then the operator must recalibrate the display of the flow meter, without the capacity to retain previously used calibration settings.